A. Field of the Invention
The present invention relates generally to communication and control networks by means of which a large number of remotely positioned controllable devices, such as circuit breakers, motor overload relays, lighting systems, and the like, may be controlled from a central or master controller over a common network line. Specifically, the present invention relates to a coupling circuit which may be employed at each remote station and at the master controller to provide bidirectional coupling to the common network line and includes amplification and limiting of a received signal. While this coupling circuit is of general application, it is particularly suitable for and will be described in connection with a low cost, multipurpose, hardware-based digital IC which is used as the basic building block in the communication and control system described in the above identified cross referenced applications. The disclosure of these cross referenced applications is hereby incorporated herein. In these cross referenced applications communication over the common network line is accomplished by means of on-off keyed carrier signals which are impressed on the common network line in a specific message format or photocol, as described in detail in said cross referenced application.
B. Description of the Prior Art
Various types of coupling circuits have heretofore been employed in communication and control systems of the prior art. Many of the prior art communication and control systems have been undirectional in nature and the coupling circuit at each remote station provides only for the reception of signals from the central controller. Such undirectional coupling circuits are shown, for example, in Eichelberger et al U.S. Pat. No. 4,091,361, Miller et al U.S. Pat. No. 4,167,786, Eichelberger et al U.S. Pat. No. 4,168,531 and Eichelberger et al U.S. Pat. No. 4,213,182. Only Eichelberger et al U.S. Pat. No. 4,091,361 involves the reception of frequency shift keyed carrier signals and this patent makes no provision for the transmission of signals from a remote station to the common network line.
Other communication and control systems have provided separate remotely located transmitter units which are associated with different groups of remote receiver-decoder stations, each of said remote transmitters being connected to a common coupler located at the central controller. Examples of each communication and control systems, which do not involve bidirectional coupling circuits associated with each remote station, are found in Feiker U.S. Pat. Nos. 4,173,754 and 4,185,272. While the Feiker patents contemplate the use of frequency shift keyed carrier signals, coupling from the separate remote transmitters is performed by means of a common directional coupler located at the central controller.
Still other communication and control systems have employed bidirectional coupling circuits for microprocessor based remote stations which are employed to control a number of mechanically latchable relays which are hard wired to each remote station. Examples of such communication and control systems are found in Miller et al U.S. Pat. Nos. 4,367,414 and 4,396,844. In these systems the transmission of signals over the common network line is accomplished by changing the impedance level on the line, either at the central controller or at one of the remotely located microprocessor based remote stations.
In the above identified Fortune et al application a carrier based bidirectional control system is disclosed which employs capacitive coupling to and from the power line, the received signal being amplified in a preamplifier circuit prior to application to a phase lock loop circuit which acts as a carrier demondulator.
When the coupling circuit is employed both to receive a modulated carrier signal from and to transmit a modulated carrier signal to a common network line, it is difficult to provide an arrangement which performs both functions efficiently and without the operation of either function affecting the other function adversely. This is particularly true where the common network line is a conventional AC power line the impedance of which may vary considerably for the high frequency carrier signals. Also, conventional power lines usually have noise and other undesired signals of different frequencies present thereon which can interfere with the transmission of plural bit messages over the power line and cause errors to be produced in one or more bits of a transmitted message. In the case of reception of a modulated carrier signal, the coupling circuit should have high selectively so that only the modulated carrier signal will be selectively amplified and all other signals, including those having frequencies close to the carrier frequency will be rejected. Furthermore, since all of the coupling circuits at remote stations are connected in parallel across the common network line, the impedance of each coupling circuit as seen by the power line should be a maximum at the carrier frequency. However, since reception of signals is most critical at the minimum voltage level of the associated amplifier, the coupling circuit should draw a minimum current at this minimum voltage level so that the highest impedance level is presented to the line for input signals of minimum voltage levels. The associated amplifier should also have high sensitivity so that this minimum voltage level is as low as possible.
In the case of transmission of a modulated carrier signal to the common network line, the maximum value of current should be supplied to the line so that a maximum number of parallel connected remote stations may be supplied with a signal of said minimum voltage level. However, due to the low impedance across the A.C. power line when a large number of remote stations is employed a low driving impedance is required which conflicts with the requirement of a high impedance input for each coupling circuit during reception of modulated carrier signals. In the transmission mode the coupling circuit should also have high efficiency and produce the desired modulated carrier signal on the power line with a minimum of distortion. The coupling circuit should also be capable of operating continuously in the transmission mode with an open circuit or short on the power line.